1. Field of the Invention
The present invention relates to a motor driving apparatus for controlling a motor that converts alternating-current (AC) power of an AC power source side into direct-current (DC) power and then further converts the DC power into the AC power to use the AC power as driving power, in particular, to a motor driving apparatus including an initial charging unit that initially charges a power storage unit provided in a DC link.
2. Description of the Related Art
Motor driving apparatuses that drive and control servomotors in machine tools, industrial machines, forging machines, injection molding machines, or various robots after converting AC power on AC power source sides into DC power, further convert DC power into AC power, and use t AC power as driving power of the motors. The motor driving apparatus includes: an AC-DC conversion unit (also referred to as a “forward converter” or a “converter”) that converts AC power supplied from an AC power source side of a commercial three-phase AC power source into DC power and output the DC power; and a DC-AC conversion unit (also referred to as a “reverse converter” or an “inverter”) that is connected to a DC link at a DC output side of the AC-DC conversion unit and converts the DC power in the DC link into the AC power for driving a motor and outputs the AC power, and controls a speed, torque, or a position of a rotor of the motor connected to an AC output side of the DC-AC conversion unit.
The DC link connecting the DC output side of the AC-DC conversion unit and a DC input side of the DC-AC conversion unit is provided with a power storage unit that can store the DC power. A DC capacitor is an example of the power storage unit.
The power storage unit needs to be initially charged in a period from immediately after start-up of the motor driving apparatus to before start of driving of the motor (i.e., before the DC-AC conversion unit starts a power conversion operation for supplying the AC power to the motor). Charge is stored in the power storage unit after completion of driving of the motor, and the charge stored in the power storage unit may be discharged to prevent an electric shock. For example, there are a method for reducing the charge in the power storage unit using self-discharge of the power storage unit and a method for consuming the charge by a resistor separately prepared.
FIG. 7 illustrates a configuration of a general motor driving apparatus including an initial charging unit. A motor driving apparatus 100 includes an AC-DC conversion unit 111 that converts AC power supplied from a commercial three-phases (an R-phase, an S-phase, and a T-phase) AC power source 3 into DC power and outputs the DC power and a DC-AC conversion unit 117 that is connected to a DC link at a DC output side of the AC-DC conversion unit 111 and converts the DC power in the DC link into the AC power for driving a motor and outputs the AC power, and controls a speed, torque, or a position a rotor of a motor 2 connected to an AC output side of the DC-AC conversion unit 117.
The AC-DC conversion unit 111 is configured as a full-bridge circuit in which a switching device and a feedback diode connected in reverse parallel to the switching device are provided in an upper arm and a lower arm in each phase. The switching devices provided in the upper arm and the lower arm are on-off controlled or all of the switching devices are turned off in response to a switching command received from a control unit 115, so that the AC power supplied from the AC power source 3 side is rectified by the diodes and converted and output as the DC power. An AC reactor 118 is connected to an AC input side of the AC-DC conversion unit 111.
An electromagnetic contactor 114 is provided on the AC input side of the AC-DC conversion unit 111 so as to open and close an electrical path between the AC power source 3 and the AC-DC conversion unit 111. In the electromagnetic contactor 114, contacts 124 are normally in a disconnected state by spring force, however, when power is supplied to a control coil 123, attraction force greater than the spring force is generated by an electromagnet, the contacts 124 connect the circuit, and the power is supplied from the AC power source 3 to the AC-DC conversion unit 111. When the power to the control coil 123 is turned off, the circuit is disconnected by the spring force, and the power supply from the AC power source 3 to the AC-DC conversion unit 111 is cut off. An opening/closing operation of an electrical path of the electromagnetic contactor 114 is controlled by an electromagnetic contactor opening/closing command output from the control unit 115.
The DC link for connecting the DC output side of the AC-DC conversion unit 111 and a DC input side of the DC-AC conversion unit 117 is provided with a power storage unit 112 that can store the DC power. In the illustrated example, the power storage unit 112 is a DC capacitor. The power storage unit 112 is initially charged in a period from immediately after start-up of the motor driving apparatus 100 (i.e., immediately after the electromagnetic contactor 114 is closed (turned on)) to before start of driving of the motor 2 (i.e., before start of a power conversion operation by the DC-AC conversion unit 117). Immediately after the start of the initial charging of the power storage unit 112 from a state in which no charge is stored therein, a large inrush current flows through the AC-DC conversion unit 111. In particular, as the power storage unit 112 has a larger capacitance, a larger inrush current is generated. As a countermeasure against the inrush current, the motor driving apparatus 100 is provided with an initial charging unit 113 between the AC-DC conversion unit 111 and the power storage unit 112. The initial charging unit 113 includes a switch unit 121 for shorting a charging resistor and a charging resistor 122 connected to the switch unit 121 in parallel. The switch unit 121 is opened (turned off) only during a period of the initial charging of the power storage unit 112 executed immediately after the start-up of the motor driving apparatus 100 and maintains a closed (turned on) state during a normal operation period in which the motor driving apparatus 100 drives the motor 2. During the initial charging period of the power storage unit 112, the switch unit 121 is opened (turned off), and the DC power output from the AC-DC conversion unit 111 flows through the charging resistor 122 and is consumed by the charging resistor 122 as heat, so that generation of an excessive inrush current is suppressed during the initial charging period. An opening/closing operation of an electrical path of the switch unit 121 in the initial charging unit 113 is controlled by a shorting command output from the control unit 115.
In order to discharge the charge in the power storage unit 112 after completion of driving of the motor, for example, a discharging unit 116 is provided. The discharging unit 116 includes a switch unit 125 for a discharging resistor and a discharging resistor 126 connected to the switch unit 125 in series. After completion of driving of the motor, the electromagnetic contactor 114 is opened (turned off), and the switch unit 125 is closed (turned on), so that the charge stored in the power storage unit 112 is consumed by the discharging resistor 126. An opening/closing operation of an electrical path of the switch unit 125 in the discharging unit 116 is controlled by a discharging command output from the control unit 115.
Other than a method of discharging using the discharging resistor as described above, there is a method of discharging using a resistance component included in a motor as described in Japanese Patent No. 5444304 and Japanese Unexamined Patent Publication (Kokai) No. 2004-357412. FIG. 8 illustrates discharging of a power storage unit in a motor driving apparatus described in Japanese Patent No. 5444304. Operations and configurations of an AC-DC conversion unit 111, a power storage unit 112, an initial charging unit 113, an electromagnetic contactor 114, a DC-AC conversion unit 117, and an AC reactor 118 in a motor driving apparatus 200 are as described above with reference to FIG. 7. In the motor driving apparatus 200 described in Japanese Patent No. 5444304, the electromagnetic contactor 114 is closed (turned on) by an electromagnetic contactor opening/closing command generated by the control unit 115, and the switch unit 121 for shorting the charging resistor is closed (turned on) by a shorting command generated by the control unit 115 so as to discharge the charge in the power storage unit 112 stored at the time of deceleration control of the motor. The DC power stored in the power storage unit 112 is converted into a reactive current by the DC-AC conversion unit 117 based on a reactive current command from the control unit 115 and supplied to the motor 2. Accordingly, the motor 2 consumes the DC power stored in the power storage unit 112 in a form of the reactive current.
Other than the above-described method, there is a method of discharging using a resistance component included in an element other than a resistor in a circuit.
For example, as described in Japanese Patent No. 5721787, there is a method for discharging a capacitor by appropriately controlling a switching operation of a switching device in a boost converter provided at a preceding stage of an inverter.
As described above, the charge is stored in the power storage unit after completion of driving of the motor, and thus the charge stored in the power storage unit may be discharged to prevent an electric shock.
However, the method using self-discharge of the power storage unit has a problem that it takes time to reduce the charge in the power storage unit.
The method for separately providing the discharging unit for discharging the power storage unit has a problem that the apparatus is enlarged and costs increase because the switch unit and the discharging resistor are needed.
The methods described in Japanese Patent No. 5444304 and Japanese Unexamined Patent Publication (Kokai) No. 2004-357412 have a problem that the motor and the DC-AC conversion unit may have cooling capacities sufficient for dealing with discharging of the charge stored in the power storage unit that enlarge the apparatus and increase costs.
The method described in Japanese Patent No. 5721787 also has a problem that the charge stored in the power storage unit is to be consumed by an element other than the discharging resistor, so that a thermal capacity and a cooling capacity of the element other than the discharging resistor are to be improved that enlarge the apparatus and increase costs.